The majority of wireless networks around the world today are 3GPP standard based supporting GSM and/or UMTS and/or LTE Radio Access Technologies. A 3GPP based User Equipment, which consists of a Mobile Equipment containing a UICC provisioned by the home Public Land Mobile Network operator, is able to distinguish one PLMN operator from another based on the unique PLMN ID, consisting solely of the MCCMNC, broadcasted by each PLMN operator and will be able to register onto its home PLMN if available. If the User Equipment is no longer able to detect the home PLMN ID, and is able to detect the PLMN ID of another PLMN operator, the UE will be able to register successfully onto a visited PLMN provided a roaming agreement exists between the home PLMN operator and the VPLMN operator.
Many people typically travel by airplane from their home country to a foreign country, and bring their UE in order to contact family, friends or colleagues back home or in the foreign country they are travelling to, upon arrival. Since almost all Airline operators require UEs to be turned off prior to takeoff, many ME manufacturers build into their ME a feature called Airplane Mode, which allows the cellular radio of the ME to be turned off when Airplane Mode is enabled/turned on, in order to comply with Airline rules and regulations. While Airplane Mode is enabled/turned on, the rest of the ME is still able to function as normal in order to allow the person travelling to enjoy listening to music, watching videos, reading eBooks or using any other functionality of their ME which does not require use of the cellular radio, during their travel while in the airplane. Once the airplane has landed safely, Airplane Mode is allowed to be disabled/turned off which turns the cellular radio back on, thus allowing the UE to attempt registration on the available VPLMN(s).
In addition to Airplane Mode, all ME manufacturers are required by 3GPP standard to implement both Automatic and Manual Network Selection modes on their MEs. MEs are typically set to Automatic Network Selection mode by default and the majority of users typically leave the ME set to this mode. Therefore, when a roaming UE's cellular radio is turned on in a foreign country with multiple VPLMNs available, for which registration is possible (due to bilateral network agreements), a UE in Automatic Network Selection mode follows the related Network Selection Procedures outlined in 3GPP TS 23.122 clause 4.4.3.1 and 4.4.3.3.
However, as per 3GPP standard, the UE is required to attempt registration on the last registered PLMN as indicated in the EF_LOCI and EF_EPSLOCI files in the UICC. Should the last registered PLMN not be available, which is typically the case when travelling to a foreign country, the UE in Automatic Network Selection mode must follow the related Network Selection Procedures outlined specified above.